Selective CO2 reduction to CH3OH over atomic dual-metal sites embedded in a metal-organic framework with high-energy radiation

The efficient use of renewable X/γ-rays or accelerated electrons for chemical transformation of CO2 and water to fuels holds promise for a carbon-neutral economy; however, such processes are challenging to implement and require the assistance of catalysts capable of sensitizing secondary electron scattering and providing active metal sites to bind intermediates. Here we show atomic Cu-Ni dual-metal sites embedded in a metal-organic framework enable efficient and selective CH3OH production (~98%) over multiple irradiated cycles. The usage of practical electron-beam irradiation (200 keV; 40 kGy min−1) with a cost-effective hydroxyl radical scavenger promotes CH3OH production rate to 0.27 mmol g−1 min−1. Moreover, time-resolved experiments with calculations reveal the direct generation of CO2•‒ radical anions via aqueous electrons attachment occurred on nanosecond timescale, and cascade hydrogenation steps. Our study highlights a radiolytic route to produce CH3OH with CO2 feedstock and introduces a desirable atomic structure to improve performance.


Point-by-point responses to the reviewers' comments
Review #1： The authors reported an unprecedented catalytic strategy to convert CO2 into CH3OH using a highenergy radiation technique combined with atomic engineering of MOFs-based catalysts, which differs from the existing thermochemical, electrolytic, and photolytic techniques. Overall, the authors made substantial efforts in this work, with insights into the high-energy radiation technique.
However, the authors did not analyze some data in depth and there are some shortcomings.
1. The renewable high-energy radiation as the energy input for the chemical transformation of CO2 and water to energy-rich fuels is interesting. Although the study is technically sound and wellperformed, it offers insufficient novelty, meaning, and conceptual advancement. Moreover, how to consider the practicality?
Response: We appreciated the interests the reviewer expressed, and also critical comments that improved the manuscript. Radiation chemists have been finding ways to tackle the environmental problems humans have created. Over the past few decades, it has been demonstrated that ionizing radiation such as electron beam (EB) and gamma-ray radiation technologies for flue gas/VOCs treatment (SOx and NOx removal), and wastewater purification can be effectively deployed to mitigate environmental degradation 1 . The implementation of EB radiation technology has been experienced in pilot plants and several industrial plants. Because of these contributions, the International Atomic Energy Agency (IAEA) has highlighted the use of radiation technology for environmental remediation as a key component in the peaceful use of nuclear technology and implemented coordinated research projects related to various aspects of this technology 2 . So, radiation chemistry, a largely unrecognized branch of chemistry, has had far-reaching effects.
Regarding radiolytic CO2 conversion, initial attempts were made during the 1970s-1990s 3-6 , which involved the direct decomposition of gaseous CO2 to CO upon irradiation. To avoid the recombination and back reactions of CO2 radiolysis, the mixture with saturated hydrocarbons such as CH4, C2H6, C3H8, and C4H10 was suggested 7,8 . Besides, the effect of γ-rays and metal ion additives has been studied using CO2-saturated solutions suspended with iron powder or zeolite 3,4 . While irradiation pathways facilitate conversion at room temperatures due to the formation of reactive intermediates, the conventional radiolytic pathways suffered from the side reactions, resulting in low radical yields (G-values, the numbers of molecules per 100 eV absorbed) and poor selectivity for specific products, in which CO is unavoidable in certain fraction. Besides, the CO2 transformation insights at irradiated interfaces were lacking. Due to these obstacles, the strategy of radiation-induced CO2 conversion was indeed once considered questionable for practical applications. Now, we have come up with fundamental new chemistry that offers a creative solution to the challenge of recycling CO2 into energy-rich CH3OH with water. This work found the combination of high-energy radiation with contemporary catalysts, i.e., CuNi SAs/UIO-66(Hf), can overcome the long-standing challenges of selectivity and energy efficiency. To the best of our knowledge, there are no reports so far on the selective and effective production of CH3OH or any liquid fuels based on the high-energy system. Importantly, we further used pulse radiolysis, along with other experimental and theoretical methods, to elucidate the fundamental radiolytic steps of CO2 activation, intermediates binding, hydrogenation, and eventual CH3OH production. As some reviewers approved, this study represents an exciting conceptual advancement, offering new opportunities for CO2 conversion beyond the existing thermal, photolytic, and electrolytic approaches, as well as beyond traditional radiolytic thinking.
We consider the practicality of our method according to the following four aspects: Firstly, we demonstrate exceptional conversion efficiency and selectivity of CH3OH under γray irradiation with 0.05 wt% CuNi SAs/UiO-66(Hf). The G value of CH3OH production was promoted to 0.9 × 10 -7 mol/J. Because CO2-to-CH3OH conversion proceeds with a six-electron transfer, the measured G value of CH3OH is two times higher G(eaq -) ~ 2.8×10 −7 mol/J in neat water radiolysis. As an example, with an absorbed dose of 10 kGy, 500 mg catalyst can produce 0.9 mmol CH3OH, and the trace of other products, such as CH4, C2H4, and formic acid, is below the detection limit of the chromatography.
Secondly, the recycling performance of catalysts has been an essential practicability consideration. We have performed more measurement cycles to verify the stability and indicated that this specific catalytic performance is maintained even under 8 cycles of irradiation treatment.
After the stability test, we conducted TEM, EDS mapping, and XPS on the samples to investigate whether any structural transformations occurred with continuous irradiation. The characterizations showed that structures of supported singles atoms remain preserved, further confirming the durable radiation catalytic property of CuNi SAs/UiO-66(Hf).
Besides, practical considerations also should include careful technical-economic analysis. To avoid the re-oxidization of • OH radicals, we used sulfite ions as a scavenger. Na2SO3 is a costeffective chemical and could be readily produced through the cases of flu gas SO2 removal process on an industry or pilot scale. The SO2 removal efficiency can exceed 80% under optimized operating conditions including pH value, liquid-gas ratio, inlet SO2 concentration, and initial Na2SO3 molar concentration. It is worth noting that industrial coal desulfurization produces approximately 10 8 tons of sulfite industrial products every year in China, which have a very low lysogenic rate and pose a risk of secondary SO2 release. By utilizing Na2SO3 as an effective •OH scavenger, our research may provide a direct source of industrial treatment for CO2 and SO2, the major component of industrial exhaust gas.
At last, the large-scale practicality was verified by our supplementary experiment on the industrial electron accelerator with high-dose rate . There are currently over 1700 EB units in commercial use and these numbers are dramatically increasing year by year worldwide. The safety regulations have been well-established. So, high-current electron accelerators and 60 Co-gamma sources are used in diverse industries to enhance the physical and chemical properties of materials and to reduce undesirable contaminants such as pathogens, and toxic byproducts. For example, it was reported in 2020 that the industrial wastewater treatment facility using EB (electron beam) technology showed the capacity to treat 30,000 cubic meters of wastewater per day 2,9 . In this regard, an EB was selected to evaluate the CH3OH generation efficiency of CuNi SAs/UiO-66(Hf). Since EB output a large amount of energy in a short time and convert sustainable electrical energy into radiation energy with high conversion efficiency (up to 80%), resulting in an impressive yield in our experiment. Specifically, we achieved a remarkable rate of 0.23 mmol/(g·min) with an irradiation time of only 2 minutes and an absorbed dose of 80 kGy using 50 mg of catalyst in a 50 mL reaction solution. As a result, we obtained a CH3OH concentration of 0.55 mmol/L. The production rate has exceeded the majority of photocatalytic and electrocatalytic synthesis efficiencies, which highlights the exceptional performance of high-energy radiation-based CO2-to-CH3OH conversion.
Overall, our work not only proposed the new conceptual radiolytic catalysis but also offers practical solutions for CO2 reduction and the utilization of greenhouse gases. The selective production of CH3OH and the integration of radiation and catalysis hold great promise for addressing global challenges related to energy, the environment, and carbon management.
2. The 60 Co γ-ray driven catalytic CO2-to-CH3OH performance of CuNi Nanoparticles/UiO-66(Hf) needs to be offered in the manuscript for comparison.
Response: We appreciate with the comments. We further synthesized CuNi nanoparticles (NPs) and UiO-66(Hf) supported CuNi NPs composites based on the reported solvent-thermal methods. Their performance in CH3OH production was conducted at identical irradiated conditions, and the results are shown in Figure 1. With dispersed CuNi NPs alone, we found that the radiolytic approach still can produce CH3OH; however, both the total yield and selectivity were far lower than the values with CuNi NPs/UiO-66(Hf) and CuNi SAs/UiO-66(Hf).On the other hand, CuNi NPs/UiO-66(Hf) displays a comparable selectivity, while the efficacy is only about 60% that of CuNi SAs/UiO-66(Hf). Second, the higher efficacy with UiO-66(Hf) matrix can be attributed to its dual role of "sensitization". In accordance with previous reports of radiation therapy and radiolytic water splitting 10-13 , UiO-66(Hf) exhibits an enhanced scattering cross-section for Auger/Compton electrons at the nanoscale interface, which significantly increases the availability of reactive electrons. In addition, the unique topology likely creates a confined environment to accelerate CO2 reduction process. In other words, catalytic reactions taking place within the nanopores of UiO-66(Hf) materials with dimensions below 1 nm experience strong confinement effects which impact the reactivity.
Third, we found atomic CuNi catalysts are superior over NPs Especially, for metal NPs like Cu, their size-dependent electronic structures will be reflected on their catalytic behavior in radiation catalysis. In the case of supported single atoms, they can be stabilized by the support through chemical bonding, especially when single atoms are anchored on MOFs supports or transition metal oxides and zeolites. Thus, those single atoms may show limited geometric transformation under reaction conditions. In the case of metal nanoparticles (>1 nm, usually with more than 40 atoms), their geometric structures are less sensitive, and usually the geometric structure of one metal nanoparticle is relatively stable, although the geometric configuration of exposed surface atoms (facet, corner, edge, metal-support interface, etc.) may change due to the environment.
3. Is the high-energy radiation catalysis universal to other bimetallic materials? The high-energy radiation powering selective CO2 reduction to CH3OH over Cu-Ni dual-metal-sites embedded MOF should be compared with other dual-metal-sites systems. Comparative performance of heterogeneous catalysts under similar reaction conditions for CO2-to-CH3OH should be added into the manuscript.
Response: thanks again for this comment. To address this important question, we have synthesized various dual-metal-sites embedded UiO-66(Hf) i.e., Cu/Co, Cu/Mn, Ni/Co, Ni/Mn. The catalytic activities in Figure 2, showed that other bimetallic materials cannot grant comparable performance during the high-energy radiation catalysis. However, the high selectivity observed in series of Cu/Ni, Cu/Co, Cu/Mn suggested the important role of metallic copper.
Our early and ongoing pulse radiolysis results found that the metallic Cu substantially extended the lifetime of CO2 •-radicals by at least 10 times compared to them on Ni surfaces through the initial surface stabilization process, laying the groundwork for the subsequent multi-electron transfer reaction. For instance, the differentiation in the transient kinetics of CO2 •-radicals adsorbed on Cu and Ni surfaces suggest diverse stabilization behavior and adsorbed structures of CO2 •-radicals, which determines the subsequent selective reduction pathway of CO2 •-radicals ( Figure 3).  Response: The durable radiation-resistance of catalysts has been an essential practicability consideration. We have performed more measurement cycles to verify the stability. The Figure 4 demonstrated that this specific catalytic performance is maintained even under 8 cycles of irradiation treatment. After the stability test, we conducted TEM, EDS mapping, and XPS on the samples to investigate whether any structural transformations occurred with continuous irradiation.
The characterizations showed that structures of supported singles atoms remain preserved, further confirming the durable radiation-resistance of CuNi SAs/UiO-66(Hf). However, as the reaction progresses, the protic solvent water can leach Ni atoms from UiO-66(Hf) over several cycles. The ICP results indicate that the content of Ni element is reduced to 0.015% after eight cycles, which also explains the absence of Ni XPS detection after cycling. This phenomenon may be one of the factors contributing to the degradation in the performance of CO2 to CH3OH. Response: Thanks for this suggestion. The evolution of hydrogenated intermediates like COOH* and CH2O* indeed is critical to the overall CH3OH production. We conduct DRIFTS spectral measurements under deep UV irradiation (193~248 nm). The results in Figure 5, clearly showed the formation of surface-bound COOH* and CH2O* within minutes. The transient peaks of these intermediates are in the consistence with previous reports 14 . Fortunately, solvated electrons could be formed through the excitation by UV light in Na2SO3 solution 14 , and the band gap of UiO-66 was also in line with the energy of deep UV light to produce the photoelectron for 6H + /6e − reduction of CO2-to-CH3OH. As shown in Figure 6a, the peak of CO2 * and HCO3 * as a carbon dioxide adsorption species was located 1692 cm -1 and 1444 cm -1 , which may act as active bodies on the catalyst surface to provide a source of CH3OH. The COOH * absorption band at 1558 cm -1 and 1648 cm -1 gradually increased along with the increase in illumination time, revealing the hydrogenation of CO2 −• radicals or CO2 on single atom sites. Furthermore, the formation of CH3O * was suggested by the stretching vibration band at 1127 cm −1 , which represents the crucial intermediates that are directly related to the CO2 to CH3OH.It should be noted that an in-situ DRIFTS setup under γ-ray irradiation has not been developed so far. The highly penetrating γ-ray irradiation often causes the malfunction of electronic devices, making it almost impossible to position the instrument close to the 60 Co γ-ray sources. Instead, we are currently updating a custom-designed DRIFTS setup coupled with X-rays.
We anticipate that these ongoing efforts will provide further insights into radiolytic CO2 reduction and relevant radiation catalysis process.

Review #2：
This paper presents exciting new results, demonstrating how CO2 gas can be radiolytically reduced all the way to methanol with reasonable efficiency in a two-metal metal-oxide-framework. This may be of great practical importance in removing CO2 from the atmosphere.
Response: Thanks very much for the comments. The g-value achieved is 0.15 micromoles/J, when scavenger for OH radicals is present. Without OH scavenger, of course, product methanol will just be re-oxidized by the OH radicals. Perhaps the authors could comment on how a practical industrialscale process might incorporate a OH scavenger for reasonable energy efficiency (the OH scavenger has to be regenerated or discarded).
Response: We greatly appreciate this comment. Among many OH scavengers, our CO2 reduction work found the best activity with Na2SO3. Sulfite is a cost-effective chemical and could be readily and Cu sites serve to catalyze the 6-electron/6-proton reduction of CO2 to CH3OH via a series of interfacial reactions involving adsorbed carbon species, starting from adsorbed CO2 •-. Another advantage of using a MOF is its huge specific surface area and its porosity towards CO2, allowing CO2 to be easily captured and channeled into the MOF framework. I believe that this work represents an exciting breakthrough that could potentially offer a novel way to use abundant highenergy radiation, that is readily available at nuclear reactors and/or electron accelerators, to convert CO2 into an energy-rich, liquid fuel. I would therefore recommend publication after the following points have been addressed: Response: We appreciated with these encouraging comments and revised the manuscript accordingly.

1.
A general comment is that no real indication is given in the manuscript as to how much methanol we could expect to generate in this process, for example per gram of MOF after a certain amount of irradiation time with a particular radiation source. Very nice data are presented in terms of G-values of the products, but I was left with no feeling of the quantity of methanol that can be generated. For example, are we talking about a few microliters of methanol per gram of MOF, or is it more than that?
Response: Thanks for this importance comment. Our earlier attempt showed that the use of a small quantity (<80 mmol/L) of UiO-66(Hf)-OH can achieve a γ-rays-to-hydrogen conversion efficiency exceeding 10% that significantly outperforms Zr-/Hf-oxide nanoparticles and the existing radiolytic H2 promoters. In UiO-66(Hf), the combination of 3D arrays of ultrasmall metal-oxo clusters and high porosity affords unprecedented effective scattering between secondary electrons and confined water, generating increased precursors of solvated electrons and excited states of water, which are the main species responsible for H2 production enhancement. In this work, the irradiation cycle test was carried out with 0.05 wt% CuNi SAs/UiO-66(Hf) in CO2 saturated aqueous solution under 4-32 kGy γ-ray irradiation. So, it is estimated, for instance with absorbed dose of 10 kGy, 0.5 gram catalysts in 1 liter water would produce 0.9 mmol (28.8 mg) CH3OH. It is noted that the absorbed dose per minute (dose rate) mainly relys on radioisotope Co-60 activity or electron beam current.
Typically, the dose rate of a electron beam varies from 40 kGy/min up to 100 kGy/s. In this case, one would expect the feasibility of CH3OH production via our newly-developed approach. Response: We have added these essential labels onto Fig. 5 and we think this makes it more intuitive to understand.
Response: We have revised this sentence to: "Compared with the reference spectrum measured in Ar-saturated 0.1 M tert-butanol solutions, in which no CO2 •-formed and only eaqremained since the scavenging of • OH and • H radical by tert-butanol (Fig. S18), the peak at 520 nm is likely assigned to adducts complex formed via the reaction between UiO-66 and eaq -." 12. Lines 326-329: It is mentioned here that "the typical transient absorption spectrum of CO2 •radical was also readily identified in the 300-400 nm region." However, CO2  13. Line 369: Define "TDOS".
Response: Thank you for the careful review, we got these two mixed up and have revised them.
Response: Thank you for your comment, we got these two mixed up and have revised them.
Supporting Information, Table S1: In the title of this table, change "Cu and loadings" to "Cu and Ni loadings".
Response: Thank you for your careful review, we got these two mixed up and have revised them.

Review #4：
The report by Hu et al. investigates the radiolytic assisted CO2 conversion to CH3OH using a bimetallic Cu-Ni MOF-based support. This is an interesting, well conducted study in an emerging research area. The authors implemented several in depth materials characterization techniques and theoretical investigations to support their findings. With that said, the novelty of this work is not as high as one would expect from the scope of this journal. Further, the concerns listed below limit the strength of the study, especially from a materials design perspective and its publication is not recommended in its current form.
Response: We appreciated the comments on materials design and characterizations. After years of effort, we are currently pioneering an emerging concept, radiolytic catalysis, which is rooted in basic radiation chemistry, aiming to solve the most pressing environmental and energy challenges, including water splitting to hydrogen, CO2 reduction, ammonia synthesis, and so on. Radiolytic catalysis is the integration of ionizing radiation and contemporary catalysts to achieve reactant conversions and product selectivity that are inaccessible to radiation or catalysts alone. While chemical transformations via radiation and catalysis are individually well-developed and optimized in many cases, efficient and effective radiolytic catalysis coupling remains primitive. Importantly, time-resolved and atomic understanding of radiolytic catalysis is further complicated by the intricate natures of radiation effects and catalysis separately.
In addition to H2 production, CO2-to-CH3OH conversion represents one of the most challenging processes because it involves six electron/H transfer. To address this challenge, we have developed a novel intermetallic catalyst synthesized via a facile radiolytic reduction process, combining copper (Cu) and nickel (Ni) at atomic level. In order to highlight these novelty and significance of our work, we will provide additional elaboration in the Introduction section. This will ensure that readers can quickly grasp the innovative aspects and the potential impact of our research in the field of radiolytic catalysis. Response: We appreciate that the reviewer pointed out the first report on the radiation sensitization role of MOF UiO-66(Hf). The sensitization ability of MOF upon X-ray irradiation produces very reactive molecules which don't travel far from the injection site-they latch on and stay right where you put them. In this case, the frameworks absorb radiation better than tissue, delivering an extra dose of radiation to the tumor. The oxidizing • OH radicals and O2 •are mainly responsible for the radiation therapy. However, this work uncocovered that secondary electron scattering in confined envrioment play a signicant role, which is rarely reported in the exsisting literature. In addition, the use of MOFs materials has never been reported on radiolytic CO2 reduction.
2. Second, this group of authors already demonstrated the use of Zr/Hf-based nanoscale UiO-66 MOFs as highly effective and stable radiation sensitizers for purified and natural water splitting under γ-ray irradiation (https://doi.org/10.1021/jacs.3c00547), thereby associating two of the main design principles implemented in this study.
Response: As mentioned by the reviewer, we reported on MOF amplified radiolytic H2 production.
The results unraveled the sensitization mechanism of MOF in the aqueous system. We found the increased radiolytic yield of excited-state water molecules and precursors of solvated electrons due to the enhanced scattering with SBU. However, since H2 is already one of the primary molecular products of water radiolysis, our earlier work did not alter the type of radiolytic products but only increased the radiolytic yield (G value) of H2, which is within the scope of conventional radiation chemistry of water.
Almost at the meantime, we made extensive attempts in catalytic CO2 reduction because the reduction of CO2 differs significantly from H2 generation. The primary products of CO2 aqueous radiolysis are carbon monoxide and oxalic acid, involving two-electron transfer reactions. It is challenging to form alcohols and alkanes through multiply electron/H transfer, a deeper conversion.
However, this study leverage the radiation sensitization effect of MOF but also, more importantly, employed MOF as matrix/support for bimetallic single-atoms to selectively regulate the types of radiolysis products, especially CH3OH. These resulted in the generation of entirely new products distinct from those produced in traditional radiolysis processes. Response: Our previous report primarily focused on the reactions and product analysis of catalystfree CO2 radiolytic reduction. Although conducted in a catalyst-free environment, the system added formate ions as reactants in addition to CO2. In contrast, the current study exclusively utilized CO2 as the only carbon source, and achieve CO2 reduction to CH3OH.
As mentioned above, the products obtained in the preliminary work mainly consisted of oxalic acid and carbon monoxide, which are typical two-electron products commonly observed in conventional radiation chemistry. We did not discover novel reaction processes overcoming the limitation until this work. However, in the present study, by incorporating of atomic Cu-Ni dualmetal-sites embedded MOF, we have achieved the selective production of a six-electron product, methanol, via radiation reduction of CO2. Compared to oxalic acid, this finding holds new promises.
Cu@UiO-66 was used in photolytic or electrolytic process, whereas it has not been investigated in distinct radiolytic system. Furthermore, we conducted additional experiments involving the loading of different metal single atoms and nanoparticles onto the MOF. The results confirmed the unique catalytic activity of the atomic copper-nickel bimetallic system, consistent with our pulse radiolysis experiments. These additional experiments further support the efficacy and potential of our proposed catalyst system. 4. In this context, the authors' claim that "The present study provides a unique and practical solution to tackle CO2 emissions and energy storage." is not fully substantiated, since this approach is not unique and also necessarily practical in the current global landscape.
Response: We have supplemented data and used industrial electron beam to strengthen the practicality of our approach. Additional illustrations were provided in revised manuscript to demonstrate the novelty.
Firstly, we have successfully demonstrated the selective reduction of CO2 to CH3OH using a bimetallic Cu-Ni catalyst embedded in a metal-organic framework (MOF). This achievement is remarkable as it involves the conversion of a six-electron transfer process, surpassing the typical two-electron reactions observed in conventional radiation chemistry.
Secondly, our study combines the synergistic effects of radiation and catalysis, enabling reactant conversions and product selectivity that are often inaccessible in radiation or catalysts alone.
By leveraging the unique properties of the Cu-Ni SAs/UiO-66(Hf) catalyst, we have achieved efficient radiolytic reduction of CO2 to CH3OH. This integration of radiation and catalytic processes expands the application scope and practical significance of radiation chemistry.
Furthermore, our work has practical implications for industrial applications. We have utilized electron beams as an energy input, which are already widely employed in industries for waste gas desulfurization and denitrification. By harnessing electron beams for CO2 reduction, we can leverage existing infrastructure and technology, making the transition to large-scale implementation more feasible. The high conversion efficiency of electron beams, coupled with the impressive yield of methanol achieved in our study, surpasses the efficiency of many conventional photocatalytic and electrocatalytic synthesis methods. This highlights the potential for electron beam-based radiolytic catalysis as a promising approach for efficient and sustainable CO2 conversion.
Overall, our work not only advances the fundamental understanding of radiolytic catalysis but also offers practical solutions for CO2 reduction and the utilization of greenhouse gases. The selective production of methanol and the integration of radiation and catalysis hold great promise for addressing global challenges related to energy, the environment, and carbon management.
5. Overall, the authors are mainly focusing on providing convincing structure-function evaluations with a focus on the catalytic pathways but lack the same level of depth for the materials characterization. Additional supporting evidence of the atomic dispersion of the dual Cu-Ni metal sites, such as determining the location via diffraction techniques/Rietveld refinement and solid-state NMR would be highly beneficial to decipher and provide additional fundamental understanding that can be then further applied for the design of superior catalytic materials.
Response: We appreciated with these comments, and performed additional 1H-NMR and neutron analysis of the samples to reveal the design principle. Interestingly, our UiO-66 synthesis process did not involve the addition of acetate (CH3COO -) as a regulator. However, the congruent presence of CH3COOobserved in our results aligns with those reported in the literature. Acetate may come from impurities such as solvents, ligands or metal salt during synthesis, which may serve as a source of coordinating ions for ligand deletion in the MOF.
These pieces of evidence were corroborated by the NMR data.
Next, we proceeded to refine the neutron diffraction data of UiO-66 (Hf  Figure 9. The content and catalytic activity of different elements given by ICP before and after radiation reduction (1M isopropanol).
Response: The material defects were characterized using NMR and neutron diffraction, and the results were as expected. Our report highlights that the solvothermal system used in this synthesis method is not purely regulated by formic or acetic acid defect regulators, and therefore defects are present. Despite this, we observed different activities of CO2 conversion to CH3OH caused by different Cu/Ni content ratios. During the catalysts synthesis process, we found that exposure of Cu and Ni precursor to radiation reduction in isopropyl alcohol aqueous solution caused a rapid decrease in the content of Ni elements. The Ni metal continued to decline after several cycles of radiation reduction of carbon dioxide in treated the CuNi SAs/UiO-66(Hf), which may lead to degradation of material properties after cycles test. The loss of Ni element after contact with protic solvent (H2O) has been reported previously 1 . Hence, while defects may provide coordination sites for Cu and Ni metals in the heat treatment stage, the Ni content is significantly reduced after radiation treatment in aqueous solution, and the performance evolution brought by the metal content may not have a clear structure-activity relationship with the defect content.
7. Also, the higher N2 uptake and associated pore volume in the Cu-Ni functionalized material is not intuitive, so this aspect needs to be justified and discussed.
Response: To collect more accurate data, we adjusted the sample quantity (700 mg) for BET adsorption and re-collected nitrogen adsorption data for both the UiO-66(Hf) carrier and the single atomic catalyst CuNi SAs/UiO-66(Hf) (Figure 10). Additionally, we added data on CO2 adsorption.
Surprisingly, we found that exposure to only 4kGy during the radiation reduction of single atoms increased the specific surface area of the support and catalyst, as well as their CO2 adsorption performance. Our comparable study on the gas adsorption performance of HKUST-1 after irradiation yielded similar results ( Figure 11). Irradiation can decompose coordination water molecules rich or other impurities in MOF, leading to increased N2 and CO2 adsorption and specific surface area. This phenomenon has been confirmed in earlier reports on MOF irradiation 6 . The presence of Cu and Ni single atoms may also enhance CO2 adsorption performance, but the extent is limited due to the low loading percentage.